1. Field of the Invention
The present invention relates to the field of optical mounts and, in particular, to a device for stabilizing a process laser beam in the presence of parasitic vibrations.
2. Description of the Related Art
An optical mount is a device which points a laser beam by controlling the orientation of an optic. The laser beam strikes the optic and is directed to a further point on the optical pathway by the interaction between the beam and the optic. The optical mount can redirect the laser beam to another point by repositioning the optic. An optical system is series of optics and their mounts.
Frequently, externally generated forces, such as reaction forces created by repositioning the optic, rotating or oscillating machinery, pedestrian traffic, vehicular traffic, and acoustic noise, cause optics in the system to vibrate undesirably. When these optics vibrate, even small vibrations can cause the laser beam to move excessively.
Generally, most externally generated vibrations occur below 200 hertz. To correct for these vibrations, closed-loop control systems for the optical mount have been developed to sense the frequency of the beam motion due to external vibration and to reposition the optic to compensate for each movement induced by the vibration.
Externally generated vibrations which can induce a vibration in the optical system can also induce a vibration in the mount if the mount has a resonant frequency in the range of the externally generated vibrations. In addition, controlling the mount to compensate for beam motion due to external vibration requires a control system bandwidth significantly greater than that of the external vibrations. If the mount has resonant frequencies in the control bandwidth, the closed-loop control system must correct for the vibration of the mount as well as the beam motion of the optical system. It is desirable to have an optical mount which will not resonate below several times the 200 hertz external vibrations. The ability to compensate for the vibration of the mount as well as for beam motion in the optical system makes the closed-loop-control system more complicated and, therefore, more expensive.
A straight-forward closed-loop control system, which corrects for vibrations below 200 hertz, requires an optical mount with a minimum resonant frequency of approximately one kilohertz. The resonant frequency of an optical mount is defined by the square root of the stiffness of the mount which repositions the optic, divided by the mass of the optic.
The optical mounts which are commercially available and which have a minimum resonant frequency of approximately 1 kilohertz have several limitations. First, they are limited to light transmitting optics having a maximum diameter of approximately 5.1 centimeters. These small mounts are available from Burleigh and Physik Instrumente.
Mounts which sacrifice the ability to transmit light but still have resonant frequencies of 1 kilohertz are available from Physik Instrumente. These small (10 cm) mounts are further limited in that the optic is attached to the mount using adhesives or other rigid fasteners. This requires custom shapes for the optic making them more expensive and prohibiting their use in other mounts.
The large optical mounts which are commercially available suffer from in-band frequencies ranging from 20 to 50 hertz. These large optical mounts, which are manufactured by Ball Aerospace, United Technologies Optical Systems, and Hughes Danbury, are also limited in that the customized optic is permanently attached to the mount.
Thus, there is a need for an optical mount which can position a light transmitting optic, has a minimum resonant frequency of approximately one kilohertz and which can interchangeably accommodate optics with diameters in excess of 10 centimeters.